target/alpha: Fix user-only floating-point exceptions

Record the software fp control register, as set by the
osf_setsysinfo syscall.  Add those masked exceptions
to fpcr_exc_enable.  Do not raise a signal for masked
fp exceptions.

Fixes: https://bugs.launchpad.net/bugs/1701835
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>